Apparatus and method for monitoring early formation of steam pop during ablation

ABSTRACT

A system for measuring real-time tissue reflection spectral characteristics during ablation includes a catheter for collecting light reflected from tissue undergoing ablation, a detection component for separating constituent wavelengths of the collected light, a quantification apparatus for generating measured light intensity data of the collected light, and a processor for analyzing the data in relation to time. A method for monitoring formation of steam pop during ablation includes delivering light to tissue, delivering ablative energy to the tissue, measuring the reflectance spectral intensity of the tissue, and observing whether the measured reflectance spectral instensity (MRSI) initially increases in a specified time period followed by a decrease at a specified rate. If the MRSI does not decrease, delivery of ablation energy continues. If the MRSI decreases within the specified time at the specified rate, formation of a steam pocket is inferred and delivery of ablative energy is decreased or discontinued.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of and claims priority to and thebenefit of U.S. application Ser. No. 13/434,454 filed Mar. 29, 2012,which is a continuation of U.S. application Ser. No. 11/552,075, filedOct. 23, 2006, which issued as U.S. Pat. No. 8,147,484 on Apr. 3, 2012,the entire contents of all which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to ablation catheters, and in particularto ablation catheters with optical monitoring of tissue for predictingsteam pops.

BACKGROUND

For certain types of minimally invasive medical procedures, real timeinformation regarding the condition of the treatment site within thebody is unavailable. This lack of information inhibits the clinicianwhen employing catheter to perform a procedure. An example of suchprocedures is tumor and disease treatment in the liver and prostate. Yetanother example of such a procedure is surgical ablation used to treatatrial fibrillation. This condition in the heart causes abnormalelectrical signals, known as cardiac arrhythmias, to be generated in theendocardial tissue resulting in irregular beating of the heart.

The most frequent cause of cardiac arrhythmias is an abnormal routing ofelectricity through the cardiac tissue. In general, most arrhythmias aretreated by ablating suspected centers of this electrical misfiring,thereby causing these centers to become inactive. Successful treatment,then, depends on the location of the ablation within the heart as wellas the lesion itself. For example, when treating atrial fibrillation, anablation catheter is maneuvered into the right or left atrium where itis used to create ablation lesions in the heart. These lesions areintended to stop the irregular beating of the heart by creatingnon-conductive barriers between regions of the atria that halt passagethrough the heart of the abnormal electrical activity.

The lesion should be created such that electrical conductivity is haltedin the localized region (transmurality), but care should be taken toprevent ablating adjacent tissues. Furthermore, the ablation process canalso cause undesirable charring of the tissue and localized coagulation,and can evaporate water in the blood and tissue leading to steam pops.The damage caused by steam pops can cause a number of problems due tothe removal and ejection of tissue, and these problems can lead tostroke or death. While a number of events can signal the occurrence of asteam pop, there are no available methods for providing advanced warningof an impending steam pop.

Thus, there is a need for a catheter capable of monitoring, inreal-time, formation of steam pocket and thereby provide early warningof impending steam pop.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method that enablereal-time optical measurements of tissue reflection spectralcharacteristics while performing ablation. The invention involves theradiation of tissue and recapturing of light from the tissue to monitorchanges in the reflected optical intensity as an indicator of steamformation in the tissue for prevention of steam pop.

In accordance with the present invention, the system includes a catheteradapted to collect light reflected from tissue undergoing ablation, adetection component that identifies and separates constituentwavelengths of collected light, a quantification apparatus forgenerating measured light intensity data for the collected light, and aprocessor that analyses the measured light intensity data in relation totime. The system may include a graphical display and/or an audio output(e.g., speaker) that provide visual and/or audio alarm when the systeminfers formation of a steam pocket in the tissue. In a more detailedembodiment, the processor of the system is adapted to infer theformation of a steam pocket likely to pop when it detects an initialincrease in the measured light intensity and a subsequent decrease at aspecified rate, and to decrease or discontinue delivery of RF energy tothe ablating catheter, which may or may not be the catheter collectinglight from the tissue.

The present invention is also directed to a method for monitoringformation of steam pocket during ablation, wherein a measuredreflectance spectral intensity MRSI versus time is analyzed. The methodincludes delivering light to tissue, delivering energy for ablation attissue and measuring the reflectance spectral intensity of the tissue,wherein observation is made as to whether the MRSI initially increasesin a specified time period followed by a decrease at a specified rate inthe MRSI. If there is no decrease in the MRSI, then delivery of ablationenergy to tissue continues. However, if there is a decrease in the MRSIwithin a specified time and at a specified rate, then the method infersthe formation of a steam pocket and decreases or discontinues thedelivery of ablative energy to tissue.

In a more detailed embodiment, the method also determines thestatistical probability of steam pop occurrence during the course ofablation. The method may also include initiating or increasing tissuecooling, such as by irrigation or infusion, to ward off steam pop if themethod has inferred a specified probability of steam pop occurrence.

The present catheter and method are designed to use light in conjunctionwith irrigation and the technology of thermal ablation. Advantageously,the light used to monitor and assess the tissue is generally notaffected by the portion(s) of the electromagnetic radiation normallyused for ablation. Moreover, the wavelength region used for monitoringand assessing steam pop also transmits through blood with minimalattenuations. In addition, the use of fiber optics to emit and receivelight is a generally temperature neutral process that adds little if anymeasurable heat to surrounding blood or tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 illustrates tissue undergoing ablation and optical monitoring inaccordance with one embodiment of the present invention.

FIG. 2 is a schematic view of incidental, scattered and reflected lightas occurring during optical monitoring in accordance with one embodimentof the present invention.

FIG. 3 is a schematic view of decreased reflected light and increasedscattered light as occurring during formation of a steam pocket.

FIG. 4 is a plot of measured spectral reflection intensity of light(“MSRI”) in arbitrary units (ordinate) against time in units of seconds(abscissa) measured during a lesion formation without steam pop in livecanine thigh muscle.

FIG. 5 is a plot of MSRI in arbitrary units (ordinate) against time inunits of seconds (abscissa) measured during a lesion formation leastevident to pop in live canine thigh muscle.

FIG. 6 a plot of representative MSRI at four different wavelengths, innanowatts of optical power (ordinate) against time in units of seconds(abscissa) measured during ablation with steam pop in live canine thighmuscle.

FIG. 7 is a plot of MSRI in arbitrary units (ordinate) against time inunits of seconds (abscissa) measured during an ablation most evident topop in live canine thigh muscle.

FIG. 8 is a plot of MSRI in arbitrary units (ordinate) against time inunits of seconds (abscissa) measured during what could be characterizedas a typical ablation session with labels corresponding to the primarydescriptive components of the MSRI.

FIG. 9 shows the results of multivariate data analysis showingtime-series attributes of rise time, drop time and average drop rate asmost predictive of steam pop.

FIG. 10 is a schematic drawing showing components of an embodiment of asystem of the present invention.

FIG. 11 is a flow chart showing a method according to one embodiment ofthe present invention wherein MRSI versus time is analyzed

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, tissue 10 is subjected to RF ablation by acatheter 12. The catheter 12 has a tip section 14 adapted for ablationin creating a lesion 16 in the tissue 10. Advantageously, the catheter12 is also adapted for optical monitoring to provide information thatcan tend to indicate the formation of a steam pocket in the lesion 16,particularly based on the formation of water vapor in the tissue. Tothat end, the catheter tip section 14 emits light that impinges on thetissue which can be reflected, scattered or absorbed. Light paths 18impinging on the tissue and light paths 20 reflected by the tissuegenerally form a scattered cloud of radiation 22 inside the tissue andin the surrounding environment whose optical intensities change as asteam pocket is formed. The changes in the optical intensities include ageneral increase usually followed by a general decrease in the measuredlight intensity. In accordance with the present invention, selectedcharacteristics of the measured light intensity curve as a function oftime can provide useful data in predicting the occurrence of steam pops.This curve does not appear to be affected by the orientation of thecatheter relative to the lesion. Illumination and detection need notoccur in the same plane. The scattering within the tissue renders bothlight paths 18 and 20 to form the scattered cloud 22; thus, the presentinvention is functional provided the illumination and detection elementsintersect the tissue-scattered cloud.

The general increase in the reflected light intensity arises fromchanges in the tissue during formation of a lesion which causes thetissue to be more reflective, as shown in FIG. 2. However, in instanceswhere a steam pop may occur, the general increase in the reflectedaverage light intensity present in the initial stages of a lesionformation by ablation may be followed by a decrease in the reflectedlight intensity. This decrease arises from vaporization of water in thetissue which collects in a steam pocket 24 within the tissue strata, asshown in FIG. 3. The steam pocket causes redirection 21 of theillumination light, due to the refractive index differences between thesteam and the tissue. Therefore, the amount of light reflected 20 to thesurface of the tissue decreases.

A catheter adapted for such optical monitoring is described in U.S.application Ser. No. 11/453,188, filed Jun. 13, 2006, and Ser. No.11/417,092, filed May 2, 2006, the entire disclosures of which areincorporated herein by reference, although it is understood by one ofordinary skill in the art that multiple catheters may be used to deliverlight to the tissue and to collect the light reflecting from the tissue.

FIG. 4 is a plot of measured spectral reflection intensity of light(“MSRI”) in arbitrary units (ordinate) against time in units of seconds(abscissa) measured during a lesion formation without steam popcollected at or near the catheter distal end shown in FIG. 1. FIG. 4 ofa “non-pop” spectrum illustrates an initial increase in spectralintensity that is generally linear from time=0 to a plateau point A atabout 40 secs. and then a decrease that is generally linear to about 300secs. FIG. 5 is another “non-pop” spectrum that is of a lesion leastevident to pop. Noteworthy is the gradual but steady asymptotic increasein spectral intensity from start of the ablation to finish.

In contrast, FIG. 6 is a plot of representative MSRI at four selectedwavelengths, in nanowatts of optical power (ordinate) against time inunits of seconds (abscissa). This figure illustrates that all of thewavelengths exhibit the same rise and fall behavior prior to pop, butthe temporal location of the peak is different at each wavelength. So,although a single wavelength is all that is necessary, the method forthe steam pop prediction as discussed further below varies depending onthat wavelength. FIG. 7 is another “pop” spectrum that is of a lesionmost evident to pop. Noteworthy is the sharp MSRI peak B at about 12secs marking the transition from the initial increase to a relativelyrapid decrease in spectral intensity.

In accordance with the present invention, a typical spectrum as shown inFIG. 8 can be decomposed into various characteristic variables,including

-   -   (a) percentage drop (peak to pop);    -   (b) time to drop or drop time (peak to pop);    -   (c) rise time (start to peak);    -   (d) maximum rate of drop (peak to pop);    -   (e) average rate of drop (peak to pop);    -   (f) percentage rise from onset (start to peak;)    -   (g) average rate of rise (start to peak); and    -   (h) last rate of drop

Tissue changes due to lesion formation by RF ablation can be spectrallycharacterized by one or more of these eight variables or variablesrelated thereto, such as increase and decrease in the MRSI and/or therate of change. And, in accordance with the present invention, certainof these eight variables and/or their related variables can be monitoredto provide an early indication of a potential steam pop.

Using simultaneous statistical consideration of the relationships amongthese eight variables (or their related variables), via the applicationof multivariate analyses, in particular, principal component analysis(PCA) and projection to latent structures (PLS) methods, as shown inFIG. 9, the following three time-series attributes are identified withthe highest correlation to steam pop prediction: (b) drop time, (c) risetime, and (e) average drop rate. Advantageously, these three attributes(and/or their related attributes) occur at such a time during lesionformation as to feasibly allow early prediction and prevention of steampops.

In accordance with the present invention, optical spectral intensity, inparticular, the measured optical spectral intensity is proficient inproviding attributes and data that can be monitored to provide earlywarning of steam pops.

With reference to FIG. 10, a system 426 for monitoring lesion opticalspectral intensity is shown. As understood by one of ordinary skill inthe art, a catheter 412 applies ablative energy to tissue 408 to formlesion 410. In accordance with the present invention, tip section 414 ofthe catheter emits light that impinges on the tissue and recaptureslight reflected by the tissue. The catheter 412 communicates with apatient interface unit (PIU) (not shown) via coupling 477 for ablationand other functions such as electromagnetic location sensing. Fluid maybe injected by a pump (not shown) to the tissue site through a luer hub490. The catheter 412 also communicates with an optical processingsystem 426. In the illustrated embodiment, the optical processing system426 includes a light source 428 that supplies a broadband (white;multiple wavelengths) light and/or laser light (single wavelength)radiation to the tip section 414 of the catheter 412 via coupling 427.Advantageously, the present invention can be performed at any wavelengththat penetrates tissue to the depth of the steam pocket. A singlewavelength of light can be used or a combination of several wavelengths.Notably, the light source 428 can be a laser, blackbody radiator, diode,or any other suitable source of illumination.

Radiation can be delivered to the catheter tip section by coupling 427.Light reflected off the tissue bearing optical intensity data from thetip section 414 is transmitted to a detection component 430 via coupling443. The detection component may comprise, for example, a wavelengthselective element 431 that disperses the collected light intoconstituent wavelengths, and a quantification apparatus 440. Theoptional wavelength selective element 431 may include optics 432, as areknown in the art, for example, a system of lenses, mirrors and/orprisms, for receiving incident light 434 and splitting it into desiredcomponents 436 that are transmitted into the quantification apparatus440.

The quantification apparatus 440 translates measured light intensitiesinto electrical signals that can be processed with a computer 442 anddisplayed graphically to an operator of the catheter. The quantificationapparatus 440 may comprise a charged coupled device (CCD) forsimultaneous detection and quantification of these light intensities.Alternatively, a number of different light sensors, includingphotodiodes, photomultipliers or complementary metal oxide semiconductor(CMOS) detectors may be used in place of the CCD converter. Informationis transmitted from the quantification device 440 to the computer 442that processes the information, processes the MSRI and provides agraphical display of the aforementioned time-series spectrum with atleast two of the three spectral characteristics of drop time, rise timeand average rate of drop (and/or their related variables). Alternately,the computer may use some combination of these spectral characteristicsin an automated algorithm that provides statistical probabilities ofsteam pop occurrence. A suitable system for use with the catheter 10 isdescribed in U.S. application Ser. No. 11/281179 and Ser. No. 11/281853,the entire disclosures of which are hereby incorporated by reference.

And, of the foregoing eight variables, the variables of rise time (orincrease in the MRSI), drop time (or decrease in the MRSI) and averagerate of drop (linearity/nonlinearity) occur at a time during ablation asto feasibly allow early prediction, if not prevention, of steam pops.

FIG. 11 is a flowchart detailing a method according to one embodiment ofthe present invention wherein MRSI versus time is analyzed. The methodincludes delivering light to tissue (Block 200) as illustrated in FIG.1, delivering energy for ablation at tissue (Block 202) to form lesionand measuring intensity of light reflected off tissue (Block 204).Initial analysis of the MRSI versus time determines if the MRSI hasdecreased following an initial increase, or peaked (Decision Block 206).If the MRSI has not peaked, then delivery of ablation energy to tissuecontinues (Block 202). However, if the MRSI has peaked, a next analysisis performed determining if the peak occurred within a specified timeperiod (Decision Block 208). If the MRSI has peaked within apredetermined or specified amount of time, then the method infers animpending steam pop and discontinues the ablation (Block 210). If thepeak occurred after a predetermined or specified amount of time, therate of drop is continuously monitored (Decision Block 209). If the rateof drop exceeds a specified amount, then the method infers an impendingsteam pop and discontinues ablation (Block 210). If the rate of dropdoes not exceed the specified amount, then delivery of ablation energycontinues. As mentioned above in relation to FIG. 6, because thetemporal location of the MRSI peak is different at each wavelength, thespecified amount of time and/or the specified amount of the rate ofDecisions Blocks 208 and 209 varies depending on the wavelength used inthe present invention.

In the method of FIG. 11, this method may include initiating orincreasing cooling of the tissue (Block 212) where the method has notyet inferred the formation of a steam pop but is approaching such aninference. As understood by one of ordinary skill in the art, cooling oftissue may include irrigation, infusion or heat exchange.

The spectral window through blood is between about 600 nm and 2000 nm,with some spots of absorption in between. A preferred range is betweenabout 650 nm and 1300 nm, although different wavelengths, particularlymonochromatic light sources, may not produce the same MSRI. that is, theintensity peak and the various slopes and drops are not the same for thesame steam pop. When using only a single wavelength, the presentinvention may perform with better sensitivity and specificity; however,any single wavelength may not clearly outperform any other singlewavelength. A more preferred wavelength may be about 900 nm.

The preceding description has been presented with reference to presentlypreferred embodiments of the invention. Workers skilled in the art andtechnology to which this invention pertains will appreciate thatalterations and changes in the described structure may be practicedwithout meaningfully departing from the principal, spirit and scope ofthis invention. Accordingly, the foregoing description should not beread as pertaining only to the precise structures described andillustrated in the accompanying drawings, but rather should be readconsistent with and as support to the following claims which are to havetheir fullest and fair scope.

1-5. (canceled)
 6. A method of predicting formation of steam pops intissue during ablation over a period of time, comprising: deliveringlight onto the tissue during the ablation; observing a measuredreflectance spectral intensity (MRSI) versus time of the tissue duringthe ablation, wherein an observation is made as to at least: an initialincrease in the MRSI; a subsequent decrease in the MRSI; and one of agroup consisting of a peak and a plateau in the MSRI, between theinitial increase and the subsequent decrease in the MSRI; and predictingon formation of a steam pop prior to formation of any steam pop based onthe observation.
 7. The method of claim 6, further comprising continuingor discontinuing the delivering light onto the tissue to preventformation of any steam pop in response to the predicting.
 8. The methodof claim 6, wherein the initial increase has a first generally linearrate and the subsequent decrease has a second generally linear rate. 9.The method of claim 8, wherein the first generally linear rate has agreater slope and the second generally linear rate has a lesser slope.10. The method of claim 6, wherein the observation includes observingthe plateau at about 40 seconds after start of ablation.
 11. The methodof claim 10, further comprising continuing the delivering light onto thetissue in response to the observation of the plateau.
 12. The method ofclaim 6, wherein the observation includes observing the peak at betweenabout 12 to 23 seconds after start of ablation.
 13. The method of claim12, further comprising discontinuing the delivering light onto thetissue in response to the observation of the peak.
 14. The method ofclaim 13, wherein the light has a wavelength between about 660 nm and1050 nm.
 15. A method of predicting on formation of steam pops in tissueduring ablation over a period of time, comprising: delivering light ontothe tissue during the ablation; observing a measured reflectancespectral intensity (MRSI) versus time of the tissue during the ablation,wherein an observation is made as to at least a generally steadyasymptotic increase in MSRI from start of ablation to finish; andpredicting no formation of steam pop prior to formation of any steam popbased on the observation.
 16. The method of claim 15, wherein theobservation occurs within about 300 seconds from start of ablation. 17.A method of predicting formation of steam pops in tissue during ablationduring a period of time, comprising: delivering light onto the tissueduring the period of time; observing a measured reflectance spectralintensity (MRSI) versus time of the tissue during ablation, whereinobservation is made as to: an increase in the MSRI; a decrease in theMSRI; and a peak in the MSRI within a duration of time from start ofablation between the increase and the decrease in MSRI; anddiscontinuing the delivering light onto the tissue to prevent formationof any steam pop in response to the observation.
 18. The method of claim17, wherein the peak occurs between about 12 and 23 seconds after startof ablation.
 19. The method of claim 17, wherein the discontinuing thedelivering light occurs within about 60 seconds or less after start ofablation.
 20. The method of claim 17, wherein the observation is alsomade as to a rise time in which MRSI reaches a peak.
 21. The method ofclaim 20, wherein the rise time spans from start of ablation to about 23seconds.
 22. The method of claim 17, wherein the observation is alsomade as to a drop time in which MRSI drops from the peak to a thresholdsteam pop formation value.
 23. The method of claim 22, wherein the droptime spans from about 23 seconds after start of ablation to about 58seconds after start of ablation.
 24. The method of claim 17, wherein theobservation is also made as to an average drop rate of the MSRI.